Types of risks

Various risks exist for humanity,
but not all are equal. Risks can be roughly categorized into six
types based on the scope (personal, regional, global) and the
intensity (endurable or terminal). The following chart provides
some examples:

The risks discussed in this article are at least Global and
Terminal in intensity. These types of risks are ones where an
adverse outcome would either annihilate intelligent life, or
permanently and drastically reduce its potential. Jamais Cascio made an alternative classification system.

Future scenarios

Many scenarios have been suggested. Some that will almost certainly
end humanity are certain to occur, but on a very long timescale.
Others are likely to happen on a shorter timescale, but will
probably not completely destroy civilization. Still others are
extremely unlikely, and may even be impossible. For example,
Nick Bostrom writes:

Some foreseen hazards (hence not members of the current
category) which have been excluded from the list on grounds that
they seem too unlikely to cause a global terminal disaster are:
solar flares, supernovae, black hole explosions or mergers,
gamma-ray bursts, galactic center outbursts, buildup of air
pollution, gradual loss of human fertility, and various religious
doomsday scenarios.

Cosmology and space

On a very long time and distance scale, the ultimate fate of the universe
is generally felt by scientists to be one that precludes the
indefinite continuation of life. There are a broad spectrum of
these predictive theories that fall in the realm of cosmology, but a long-established and
widely-accepted notion is the Heat death of the universe. Most
notions involve time periods much greater than the age of the universe, around 13 billion
years.

At the latest, in about 5 billion years, stellar evolution predicts our sun will exhaust its core hydrogen and become a red giant. In doing so, it will become thousands
of times more luminous. As a red giant, the Sun will lose roughly
30% of its mass, so, without tidal effects, the Earth will be in an
orbit from the Sun when the star reaches its maximum radius.
Therefore, the planet is thought to escape envelopment by the
expanded Sun's sparse outer atmosphere, though most (if not all)
existing life would have been destroyed by the Sun's proximity to
Earth. However, a more recent simulation indicates that Earth's
orbit will decay due to tidal effects and drag, causing it to enter
the red giant Sun's atmosphere and be destroyed. The Earth will
likely be dragged into the Sun when it becomes an enlarged red giant by no later than about 7.6 billion
years; before actual collision with the sun, the oceans would
evaporate, and Earth could be destroyed by tidal forces. Alternatively, if the Sun shrinks
to a white dwarf before consuming Earth,
the Earth would be too frigid to sustain life.

Meteorite impact

In the timeframe of the geologically recent history of the Earth, say, 100 million
years, several large meteorites have hit
Earth. The Cretaceous-Tertiary
asteroid, for example, is theorized to have caused the
extinction of the dinosaurs. If such an
object struck Earth it could have a serious impact on civilization.
It is even possible that humanity would be completely destroyed;
for this, the asteroid would need to be at least 1 km (0.62
miles) in diameter, but probably between 3–10 km (2–6 miles).
Asteroids with a 1 km diameter impact the Earth every 500,000
years on average. Larger asteroids are less common. The last large
(>10 km) impact happened 65 million years ago.
So-called Near-Earth asteroids
are regularly being observed.

A star passage that will cause an increase of meteorites is the
arrival of a star called Gliese 710. This
star is moving on a near collision course with the Solar System and will likely pass within 1.1
light years from the Sun in 1.4 million
years. Some models predict that this will send large amounts of
comets from the Oort cloud to the Earth.
Other models, such as the one by García-Sánchez, predict an
increase of only 5%.

Other cosmic threats

A number of other scenarios have been suggested. Massive objects,
e.g., a star, large planet or black hole, could
be catastrophic if a close encounter occurred in the solar system.
(Gravity from the wandering objects might disrupt orbits and/or
fling bodies into other objects, thus resulting in meteorite
impacts or climate change. Also, heat
from the wandering objects might cause extinctions; tidal forces
could cause erosion along our coastlines.) Another threat might
come from gamma ray bursts.
Both are very unlikely.

Still others see extraterrestrial
life as a possible threat to humankind; although alien life has
never been found, scientists such as Carl
Sagan have postulated that the existence of extraterrestrial
life is very likely. In 1969, the "Extra-Terrestrial Exposure
Law" was added to the Code of Federal Regulations (Title 14,
Section 1211) in response to the possibility of biological
contamination resulting from the US Apollo Space Program. It was
removed in 1991. Scientists consider such a scenario technically
possible, but unlikely.

In April
2008, it was announced that two simulations of long-term planetary
movement, one at Paris Observatory and the other at University of
California, Santa Cruz indicate a 1% chance that Mercury's orbit could be made unstable by
Jupiter's gravitational pull sometime during
the lifespan of the sun. Were this to happen, the
simulations suggest a collision with Earth could be one of four
possible outcomes (the others being colliding with the Sun,
colliding with Venus, or being ejected from the solar system
altogether). If this were to happen, all life on Earth would be obliterated and the impact may displace
enough matter into orbit to form another moon. Note that an
asteroid just 15 km wide is said to have destroyed the
dinosaurs; Mercury is some 5,000 km in
diameter.

Earth

Global pandemic

A less predictable scenario is a global pandemic. For example, if
HIV were to mutate and become as transmissible
as the common cold, the consequences
would be disastrous. This particular scenario would also contradict
the observable tendency for pathogens to become less fatal over
time as a function of natural
selection . A pathogen that quickly kills its hosts will not
likely have enough time to spread to new ones, while one that kills
its hosts more slowly or not at all will allow carriers more time
to spread the infection, and thus likely outcompete a more lethal
species or strain. A real-life example of this process can be found
in the historical evolution of syphilis
towards a less virulent form. Also, as a virus mutates and
becomes easily transmittable it often gives up much of its
virulence in the process. This is not to say that a highly
destructive and highly transmissible disease is not possible.
Ebola, for example, is highly contagious and
up to 90% fatal; the only reason it has not caused a worldwide
crisis is because outbreaks usually occur in rural Africa. Of
course, a pandemic resulting in human extinction need not arise
naturally; the possibility of one caused by a
deliberately-engineered pathogen cannot be ruled out.

Megatsunami

Another possibility is a megatsunami. A megatsunami could,
for example, destroy the entire east coast of the United States of America. The coastal areas of the entire world could
also be flooded in case of the collapse of the West Antarctic Ice Sheet. While
none of these scenarios are likely to destroy humanity completely,
they could regionally threaten civilization. There has been
one recent
high-fatality tsunami, although it was not large enough to be considered
a megatsunami.

Climate Change & Global Warming

Climate change is any long-term
significant change in the expected patterns of average weather of a
specific region (or, more relevantly to contemporary
socio-political concerns, of the Earth as a whole) over an
appropriately significant period of time. Climate change reflects
abnormal variations to the expected climate within the Earth's
atmosphere and subsequent effects on other parts of the Earth, such
as in the ice caps over durations ranging from decades to millions
of years. According to the UN’s Office for the Coordination of
Humanitarian Affairs (OCHA), climate disasters
are on the rise. Around 70 percent of disasters are now climate
related – up from around 50 percent from two decades ago. These
disasters take a heavier human toll and come with a higher price
tag. In the last decade, 2.4 billion people were affected by
climate related disasters, compared to 1.7 billion in the previous
decade and the cost of responding to disasters has risen tenfold
between 1992 and 2008. Destructive sudden heavy rains, intense
tropical storms, repeated flooding and droughts are likely to
increase, as will the vulnerability of local communities in the
absence of strong concerted action.

Ice age

In the history of the Earth, 12 ice ages have occurred. More ice
ages will almost certainly come at an interval of 40,000–100,000
years. This would have a serious impact on civilization, because
vast areas of land (mainly in North
America, Europe, and Asia) could become uninhabitable. It would still be
possible to live in the tropical regions, but with possible loss of
humidity/water. Currently, the world is existing in an interglacial
period within a much older glacial event. The last glacial
expansion ended about 10,000 years ago, and all civilizations, save
a few hunter-gatherer populations, have come into existence during
that time.

World population and agricultural crisis

The 20th century saw a rapid increase in human population due to medical advances and
massive increase in agricultural productivity made by the Green Revolution. Between 1950 and 1984, as
the Green Revolution transformed agriculture around the globe,
world grain production increased by 250%. The Green Revolution in
agriculture helped food production to keep pace with worldwide
population growth or actually
enabled population growth. The energy for the Green Revolution was
provided by fossil fuels in the form of fertilizers (natural gas), pesticides (oil), and hydrocarbon fueled irrigation. David Pimentel, professor of ecology and
agriculture at Cornell
University, and Mario Giampietro, senior researcher at the
National Research Institute on Food and Nutrition (INRAN), place in
their study Food, Land, Population and the U.S.Economy the maximum U.S.
population for a sustainable
economy at 200 million. To achieve a sustainable economy and avert
disaster, the United
States must reduce its population by at least one-third,
and world population will have to
be reduced by two-thirds, says the study.

The authors of this study believe that the mentioned agricultural
crisis will only begin to impact us after 2020, and will not become
critical until 2050. Geologist Dale
Allen Pfeiffer claims that coming decades could see spiraling
food prices without relief and massive starvation on a global level such as never
experienced before.

Supervolcano

When the
supervolcano at Yellowstone last erupted 640,000 years ago, the magma and ash
ejected from the caldera covered most of the United States west of the
Mississippi river and part of northeastern Mexico. Another
such eruption could threaten civilization. Such an eruption could
also release large amounts of gases that could alter the balance of
the planet's carbon dioxide and cause a runaway greenhouse effect,
or enough pyroclastic debris and other material may be thrown into
the atmosphere to partially block out the sun and cause a volcanic winter, as happened in 1816, the
Year Without a Summer. Such an
eruption may cause the immediate deaths of millions of people
several hundred miles from the eruption, and perhaps billions of
deaths worldwide due to the failure of the monsoon , as well as destruction of the "American
breadbasket", causing starvation on a massive scale.

Humanity

Some threats for humanity come from humanity
itself. The scenario that has been explored most is a nuclear war or another weapon with similar possibilities.
It is difficult to predict whether it would exterminate humanity,
but very certainly could alter civilization, in particular if there
was a nuclear winter.

It has been suggested that learning
computers that rapidly become superintelligent may take unforeseen
actions or that robots would
out-compete humanity. Because of its exceptional scheduling and
organisational capability and the range of novel technologies it
could develop, it is possible that the first Earth
superintelligence to emerge could rapidly become very, very
powerful. Quite possibly, it would be matchless and unrivalled:
conceivably it would be able to bring about almost any possible
outcome, and be able to foil virtually any attempt that threatened
to prevent it achieving its desires. It could eliminate, wiping out
if it chose, any other challenging rival intellects, alternatively
it might manipulate or persuade them to
change their behaviour towards its own interests, or it may merely
obstruct their attempts at interference.

Biotechnology could lead to the
creation of a pandemic, Nanotechnology could lead to grey goo in which out-of-control self-replicating robots consume all living
matter on Earth while building more of
themselves - in both cases, either deliberately or by accident.It
has also been suggested that physical scientists might accidentally
create a device that could destroy the earth and the solar system.
Another kind of accident is the Ice-9 Type Transition, in which
our planet including everything on it becomes a strange matter
planet in a chain reaction. Some do not view this as a credible
scenario.

It has been suggested that runaway global
warming might cause the climate on Earth to become like
Venus, which would make it uninhabitable. In
less extreme scenarios it could cause the end of civilization.
According
to a UN climate report, the Himalayan glaciers that are the sources of Asia's biggest rivers - Ganges, Indus, Brahmaputra, Yangtze, Mekong, Salween and Yellow - could disappear by 2035 as temperatures
rise. Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers.
India, China, Pakistan, Bangladesh, Nepal and Myanmar could
experience floods followed by droughts in
coming decades.In India alone, the
Ganges provides water for drinking and farming for more than 500
million people. The west coast of North America, which gets much of its water
from glaciers in mountain ranges such as the Rocky Mountains, Cascade Mountains and Sierra Nevada, also would be affected.
According
to the California Department
of Water Resources, if more water supplies are not found by
2020, California residents will face a water
shortfall nearly as great as the amount consumed
today.Directly linked to observed increases in the intensity
and frequency of natural disasters, global warming and climate
change are now considered key drivers behind rising global
humanitarian and emergency relief needs. According to the UN’s
Office for the Coordination of Humanitarian Affairs (OCHA), climate disasters are on the rise. Around 70
percent of disasters are now climate related – up from around 50
percent from two decades ago. These disasters take a heavier human
toll and come with a higher price tag. In the last decade, 2.4
billion people were affected by climate related disasters, compared
to 1.7 billion in the previous decade and the cost of responding to
disasters has risen tenfold between 1992 and 2008. Destructive
sudden heavy rains, intense tropical storms, repeated flooding and
droughts are likely to increase, as will the vulnerability of local
communities in the absence of strong concerted action.

Approximately 40% of the world's agricultural land is seriously
degraded. In Africa, if current trends of
soil degradation continue, the continent mightbe able to feed just
25% of its population by 2025, according to UNU's Ghana-based Institute for
Natural Resources in Africa.

Other scenarios

Fossil Fuels attain a level of scarcity before an economically
viable replacement is devised, leading firstly to economic strain,
followed by the collapse of modern agriculture, then to
mass-starvation.

There is some speculation that global
warming could, via a shutdown or slowdown of the thermohaline
circulation, trigger localized cooling in the North Atlantic and
lead to cooling in that region. This would affect in particular areas like
Ireland, the Nordic
countries, and Britain that are warmed by the North Atlantic drift.

Some scenarios of simultaneous ecological (food & water
production) and economical (see f.e. below) collapses with
overpopulation are presumed to lead to a global civil war, where
the remaining habitable areas are destroyed by competing humans (so
called 'Mad Max'-scenario).

As of late 2007, increased farming for use in biofuels, along with world oil
prices spiking to more than $140 per barrel, had pushed up the
price of grain used to feed poultry and dairy cows and other
cattle, causing higher prices of wheat (up 58%), soybean (up 32%),
and maize (up 11%) over the year. Food riot have
recently taken place in many countries across the world. An
epidemic of stem rust on wheat caused by race Ug99 is
currently spreading across Africa and into
Asia and is causing major concern. Scientists
say millions of people face starvation.

Experimental accident

Investigations in nuclear and high energy physics, such as the
Trinity test
and more recently with the Large Hadron Collider,
theoretical chain-reaction global disasters triggered by these
unusual conditions were worried about by some but have not yet
occurred.

Historical futurist scenarios

Sir Isaac Newton (1642-1727)
studied old texts and surmised that the end of the world would
happen no earlier than 2060, although he was reluctant to put an
exact date on it.

Many believe that the Mayan
civilization's Long Count
calendar ends abruptly on December
212012. This misconception is due to the
Mayan practice of using only five places in Long Count Calendar
inscriptions. On some monuments the Mayan calculated dates far into
the past and future but there is no end of the world date. There
will be a Piktun
ending (a cycle of 13 144,000 day Bak'tuns) on December 21, 2012. A
Piktun marks the end of a 1,872,000 day or approximately 5125 year
period and is a significant event in the Mayan calendar. However,
there is no historical or scientific evidence that the Mayas
believed it would be a doomsday.